This invention relates to the processing of deasphalted residual petroleum charge stocks and in particular to the visbreaking of such charge stocks.
2. Description of the Prior Art
Visbreaking, or viscosity breaking, is a well known petroleum refining process in which reduced crudes are pyrolyzed, or cracked, under comparatively mild conditions to provide products having lower viscosities and pour points thus reducing the amounts of less-viscous and more valuable blending oils required to make the residual stocks useful as fuel oils. In a typical visbreaking process, the crude or resid feed is passed through a heater and thereafter into a reaction chamber operating at from about 800.degree. to about 975.degree. F. and at about 50 to about 1000 psig. Light gas-oil is injected to lower the temperature of the effluent to within about 830.degree. to about 850.degree. F. Cracked products from the reaction chamber are introduced into a flash distillation unit with the vapor overhead being separated in a fractionating column into a light distillate overhead product, e.g., gasoline and light gas-oil bottoms, and the liquid bottoms being separated in a vacuum fractionating column into heavy gas-oil distillate and residual tar. Examples of such visbreaking methods are described in Beuther et al., "Thermal Visbreaking of Heavy Residues", The Oil and Gas Journal. 57:46, Nov. 9, 1959, pp. 151-157; Rhoe et al., "Visbreaking: A Flexible Process", Hydrocarbon Processing, January 1979, pp. 131-136; and U.S. Pat. No. 4,233,138 all of which are incorporated herein by reference. Heretofore, visbreaking has had only a limited efficiency when processing charge stocks containing asphaltenes. In conventional visbreaking of such charge stocks a sediment in the form of coke is formed which has the tendency to plug the visbreaker reactor, shorten production runs and result in unacceptably lengthy periods of down time. It has now been observed that visbreaking of such asphaltene-containing charge stocks is greatly enhanced when the petroleum charge stocks are deasphalted prior to entering the visbreaking unit.
Solvent deasphalting is a well-known operation in refineries. Primarily, deasphalting is used to separate a petroleum resid into a relatively high quality fraction (deasphalted oil or maltenes) and a lower quality fraction (asphalt or alphaltenes). Many solvents and solvent combinations have been suggested for this process. Most commonly, light hydrocarbon solvents or paraffins containing 3 to 8 carbon atoms in the molecule such as propane, propylene, butene, butane, pentene, pentane and mixtures thereof and used either alone or in admixture with other solvents such as ketones, liquid SO.sub.2, cresol and diethyl carbonate. Typical or prior art deasphalting processes is the process described in U.S. Pat. No. 2,337,448 the entire contents of which are incorporated herein by reference. The prior art also covers examples of processes which incorporate visbreaking and deasphalting in that order.
U.S. Pat. No. 2,875,149 teaches a method for recovering asphaltenic constituents from a residual asphaltenic oil by visbreaking the residual oil and deasphalting the product formed.
U.S. Pat. No. 3,532,618 teaches a process for producing a pour point depressant for shale oil by contacting a deasphalting solvent under deasphalting conditions with the shale oil which has been previously hydro-visbroken.
By visbreaking a previously deasphalted resid followed by reblending the previously recovered asphaltene fraction, a high quality petroleum product of low viscosity and pour point is formed. In addition the problems associated with visbreaking asphaltene-containing resid feeds are eliminated by the removal of the asphaltenes. The removal of asphaltenes from the resid product prior to visbreaking eliminates coke formation which is largely due to condensation type of reactions of the asphaltenes. Therefore, the visbreaker will not be subjected to the problems associated with coke formation, such as reactor plugging, in the processing of the resid feed. Therefore the maltene fraction can be visbroken to a much higher severity than the whole resid feed and then can be recombined with asphaltenes resulting in a useful petroleum product of much lower viscosity and pour point and more compatible which in turn reduces the cutter stock requirements compared to conventional visbreaking.